Optimizing Apache, PHP, and MySQL is beyond the scope of this book, but if you are expecting large amounts of traffic or simply want to squeeze as much power as possible out of your existing server setup, then the topic is worth investigating.
Many webmasters have found that using lighttpd or nginx instead of Apache offers a performance boost. Also, tweaking the settings for MySQL and PHP may help you. If you need advice about how to do this, I would recommend contacting your web host.
Some tweaks are possible only on a VPS hosting plan or better. It is possible to run WordPress MU and BuddyPress on a shared hosting plan; however, if you find yourself hitting the limits of such a plan, the best option is to upgrade to a VPS. If you have difficulty on a smaller VPS plan (for example, 256MB RAM), again an upgrade may be the easiest fix. However, for plans bigger than that, you can most likely earn yourself some headroom by optimizing the server's setup.
Swapping from...